HG you are right inpart, I remember that thread.
Basically you have a first interview. You could be invited to an interview by just one of your preferred HM's meaning the others aren't interested. If that HM likes you, you get added to their list, regardless of wether they were your first, second or third choice you're just glad to be on a list at this stage
If you're lucky enough, you may get invited to interview by two HM's or all three. If all like the candidate, NOW, you are in the enviable position to choose between the three. This was the case with the poster you spoke of.
You may be invited to interview by just one HM and if he doesn't like the candidate, doesn't get added to his list, although can still come for pre test but unlikely to get in.
You may be very unlucky and may not be invited at all for interview, which means you're still eligible for the pre test, but you're sort of wasting your time.
English is not my first language so I may not be explaining this very well. But this is kind of how it works, although it's all changed from this year now with ISEB also being introduced into the process.
Much prefer Eton's process and Winchester's from what I've read here.